Design Brief-Underfloor Air Distribution

7/28/2019 Design Brief-Underfloor Air Distribution

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Summary

Underfloor air distribution and access floor systems provide

numerous advantages to commercial building owners and

occupants compared to traditional overhead air distribution

systems with pipe and wire data, telecommunications, and

electrical distribution systems. Underfloor air systems can

provide energy savings while improving comfort and indoor air

quality in many applications. The additional costs of the access

floor system are partially offset by savings in wiring and HVAC

installation costs. In buildings where frequent remodeling is

required, savings in remodeling costs alone can easily pay for the

system. Improvements in occupant health and comfort due to

improved indoor air quality can increase employee productivity

and company profitability.This design brief is an introduction

to underfloor air and access floor systems, and addresses the

following topics:

s Displacement ventilation and hybrid underfloor systems

s Energy savings and indoor air quality improvement

s Access f loor system design and construction

s Economics of combined underfloor air and access

floor systems

s Comfort and productivity issues

s Applications of underfloor air and access f loor systems

energydesignresources

Underfloor air distribution and

access floor systems can provide

energy savings, improved indoor

air quality, and a technology

ready environment for todays

commercial buildings.

contents

Introduction 2

Underfloor Air Distribution 3

Indoor Air Quality 5

Energy Impacts 7

Access Floor Systems 10

Comfort Control and Productivity 14

Economics 17

Applications 18

Design Considerations 19

Design Methods 23

For More Information 25

Notes 27

UNDERF LOOR A IR D I STR IBUT ION AND ACCESS F LOORS

design brief


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page 2 underfloor air distribution and access floors

Buildings designed with underfloor air distribution and access

floor systems can offer the enhanced level of energy efficiency,

comfort, productivity, flexibility, and improved infrastructure

demanded by building owners and occupants in todays market.

Introduction

What Is Underfloor Air Distribution?

Underfloor air distribution systems are a general class of air

distribution systems that deliver air through diffusers in the

floor, with the intent of maintaining comfort and indoor air

quality levels only in the occupied lower portion of space.

These systems are increasingly popular alternatives to the

traditional overhead,or fully-mixedsystems, which attempt tocondition the air in the whole volume of space. Underfloor

systems provide unique opportunities for energy savings,

enhanced comfort control, and improved indoor air quality.

What Is an Access Floor?

An access floor is a modular system of architectural f loor panels

installed on pedestals above the structural floor to create an

easily accessible underfloor space. Traditionally, access floor

systems have been widely used in clean rooms and in spaces

with large amounts of electronic equipment, such as control

rooms and computer rooms. With the arrival of the

technologically laden office environment, demand for access

floors is rising rapidly. Owners need their buildings to be

technology-ready, with ample power, voice, and data services

that are easily accessible and reconfigurable.

Why Are These Systems Important to

Building Designers and Owners?

Underfloor air distribution can provide better comfort, higher

indoor air quality, and lower energy costs. An access floor

system provides flexibility in space management as well as easy

maintenance of power, voice, and data wiring. Integrating the


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page 3underfloor air distribution and access floors

underfloor air distribution system with the access floor creates

the opportunity for better management of communications and

data infrastructure with improved HVAC. This synergistic

combination of building systems gives the building owner an

integrated design solution that can provide a substantial returnon investment over the life of the building.

Underfloor Air Distribution

Underfloor air distribution systems introduce air at the floor

level, with return grilles located near the ceiling.The space is

divided into two zones, an occupied zone extending from the

floor to head level, and an unoccupied zone extending from the

top of the occupied zone to the ceiling. The systems are

designed to condition the lower occupied zone only;

temperature conditions in the upper zone are allowed to float

above normal comfort ranges.To avoid occupant discomfort, air

is introduced into the space between 65F and 68F.

In contrast, traditional overhead ventilation systems supply and

return air at the ceiling. The system produces a large single

zone of fully-mixed, room-temperature air. Using a liquid and

beaker analogy, Figure 1 is a simplified illustration of the

difference between an underfloor system and a conventional

fully-mixed system. In the underfloor system, cool liquid

introduced from below flows through the occupied zone,

picking up heat and contaminants and pushing them into the

unoccupied zone above. In the fully-mixed system, cooler liquid

delivered from above mixes with all the liquid to maintain a

constant temperature throughout the beaker. This dilutes

contaminants but does not effectively purge them.

Underfloor air distribution systems fall into two general categories

distinguishable from one another by the temperature and

velocity profiles they create in the occupied space. The first

type is a displacement ventilation system; the second type is

a hybrid underfloor system.

The upper figure shows a beaker filled

and emptied from the top,much like an

overhead mixed ventilation system.The

lower figure shows a beaker filled with

cool water from the bottom, with warm

water exiting from the top.

Figure 1: Underfloor ventilationbeaker analogy

Displacement System

Source: Healthy Buildings Internationa

Mixing System


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Displacement Ventilation

Displacement ventilation systems deliver air at floor level into

the space at very low velocity, typically less than 50 feet per

minute (fpm). At this velocity, the air coming out of the diffuser

can barely be felt, and the fresh air pools onto the floor. The

system produces two distinct zones of air, one characterized by

stratified layers of relatively cool and fresh air, the other by fairly

uniform hot and stale air. The vertical flow profile in the

lower zone can be generally described as upward laminar flow,

or plug f low. The effect of the plug flow is to displace the

hot stale air into an area well above the breathing level of the

occupants, giving occupants the benefit of breathing

significantly higher-quality air. The displacement effect is

augmented by the presence of heat sources within the

occupied space, as shown in Figure 2. The thermal plume

created by a heat source has the effect of enhancing the airflow

around the source, thereby improving overall heat removal.The

plume is inherently advantageous in applications where air

contaminants and heat are linked to the same source.

Draft is usually not an issue in spaces served by displacement

ventilation systems, but the temperature difference between the

floor and head levels (the temperature gradient) is an important

design issue. Displacement ventilation systems are generally

applied to spaces that require cooling during occupied hours.

They do not function well as heating systems; when heating is

required, it is generally supplied by a separate system.

Conventional European design practice limits the use of

displacement ventilation systems to spaces with peak cooling

loads of 12 Btu/hr-ft2 or less. In spaces with higher cooling

loads, radiant cooling systems are used in combination with

displacement ventilation. Recent U.S. research suggests that

displacement ventilation systems can be applied to spaces with

cooling loads up to 38 Btu/hr-ft2.1

In a displacement ventilation system,

cool air pools on the floor and rises

slowly as it picks up heat. Heat sources

create plumes that improve air

circulation.

Figure 2: Displacementventilation airflow

Source: Architectural Energy Corporation


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Hybrid Underfloor System

The second general type of underfloor air distribution system

can be characterized as a hybrid underfloor system, a

combination of displacement ventilation and conventional

mixing systems. Like the displacement ventilation system, the

hybrid underfloor system attempts to condition only the

occupied lower portion of space, producing two distinct zones

of air, one cool and relatively fresh, the other hot and stale.

Unlike the displacement ventilation system,however, the hybrid

underfloor system aims to reduce the stratification in the

occupied lower portion by delivering air at higher velocity (200

to 400 fpm).This results in a more mixed and turbulent vertical

flow profile and a smaller temperature gradient. While hybridunderfloor systems may more or less reduce the comfort

problems associated with an excessive temperature gradient,

they usually create small subzones of excessive draft called

clear areasthat occupants need to avoid (Figure 3). Moreover,

hybrid underfloor systems may not provide as dramatic an

improvement in air quality at the breathing level as do

displacement ventilation systems. Hybrid underfloor systems

can handle higher cooling loads than displacement ventilation

systems; the cooling load capacity is limited only by the number

of diffusers used and the number of clear areas created.

Indoor Air Quality

Underfloor air distribution systems, particularly displacement

ventilation systems, provide a natural advantage over

conventional overhead systems due to their ability to efficiently

move stale and contaminated air out of the occupied space.The

pollution level of the air at breathing level is always lower in

spaces served by displacement ventilation systems. This

phenomenon is a reflection of the ventilation effectiveness of

the system. Simply stated, ventilation effectiveness indicates

how efficiently the system is able to move contaminated air

from the room to the return air duct. From an engineering

Return Outlet

Displacement Zone

Clear Zone

Supply Diffuser

Mixing Zone

The airflow patterns in a hybrid

underfloor system are shown below.

The supply diffuser entrains room air,

creating a layer of mixed air in the

occupied zone.To prevent drafts,

occupants should avoid the clear area.

Figure 3: Hybrid underfloorsystem airflow

Source: Loudermilk, 1999


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perspective,ventilation or air-changeeffectiveness is defined as

the ratio of contaminant concentration in the return air to the

contaminant concentration in the breathing zone of the room.

Conventional overhead, fully-mixed systems are designed in

hopes of providing exactly 100 percent ventilation

effectiveness throughout the room. Since the air in the room isassumed to be perfectly mixed, the concentration of the return

air should be the same as that of the room air; hence the ratio

of contaminants in the return air to contaminants in the room air

is one. Spaces served by overhead systems are prone to common

comfort-related problems such as short-circuiting, dumping,

and dead spots. Short-circuiting occurs when a portion of the

air discharged from the overhead diffuser never reaches the

occupied lower space, but instead simply flows directly to the

return grille.This can happen in overhead systems if airflow is

warmer than room air.Dumping occurs when the velocity of the

supply air is too slow to induce mixing and the cold air simply

falls, or dumps, to the floor. This can happen when the air

velocity and/or supply air temperatures are lower than the

diffuser can handle. Dead spots occur when the location of

Ventilation effectiveness as a function of the difference between the supply

temperature and the room temperature for overhead and displacement ventilation

systems is shown below. Note that the ventilation effectiveness of displacement

systems is higher than 100 percent during cooling operation.

Figure 4: Ventilation effectiveness with overhead anddisplacement ventilation

Theory

Actual

1.5

1.25

1.0

0.75

0.5

0.25

10F 8 6 4 2 +2 + 4 + 6 +8 +10F

Supply air temp. (T1)

Room

temp.

Room

temp.

Ventilationefficiency(V

E)

V=CEx

CBZ

CEx

T1

CBZ

1.5

1.25

1.0

0.75

0.5

0.25

-10F -8 -6 -4 -2 +2 +4 +6 +8 +10F

Supply air temp. (T1)

Thermal displacement ventilation: supply air low, exhaust high Mixing ventilation: supply air and exhaust high

VentilationEfficiency(V

E)

VE=CEx

CBZ

CExT1

CBZ

Source: Healthy Buildings International


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partitions and furniture relative to the diffuser location inhibit

complete mixing in the space creating dead spots of air.

In the occupied lower zone of a displacement ventilation

system, air is displaced, not mixed, and allowed to stratify. The

flow pattern established is called plug flow, which is very

efficient at purging contaminants from the occupied zone. In

displacement ventilation systems, ventilation effectiveness

around the knees can be as high as 200 percent, while at the

breathing level it can be as high as 120-150 percent (Figure 4).

Hybrid underfloor systems, by contrast, do not necessarily

provide the dramatic increase in ventilation ef fectiveness that

displacement ventilation systems offer, though they are

definitely an improvement. They do not produce the plug

flow that is so efficient at pushing pollution out of the space.

Instead they promote varying amounts of mixing in the

occupied space by inducing the recirculation of room air.

Mixing in lieu of plug flow intrinsically limits ventilation

effectiveness. Hybrid underfloor systems generally provide

100 percent ventilation effectiveness in the cooling mode, the

same as a well-designed overhead system. However, the fact

that occupants can control the airflow rate to their spaceimproves the perceived quality of air, so overall acceptance of

the system is greater than with overhead systems.

Energy Impacts

System Design Flow Rate

System flow rates are generally higher in underfloor systems than

in conventional overhead systems, since warmer supply air

temperatures (65F vs.55F for a conventional system) are used,resulting in a smaller difference between the supply air and the

desired room temperature. However, because comfort conditions

in an underfloor system are maintained in the lower occupied

zone only, the increase in supply air flow rate is less than one

might expect.A comparison of design flow rates for a typical office

The ventilation effectiveness of

displacement ventilation systems can

approach 150 percent at the breathing

level.

Underfloor air distribution systems

can be very energy-efficient, even at

increased flow rates.


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space is shown in Figure 5. The differences in flow rates for each

system are fairly modest,on the order of 5 to 20 percent.

Although underf loor systems generally operate at higher flow

rates than conventional overhead systems, they typically

consume less energy than a conventional overhead system due

to a number of factors, including:

1. Reduction in air distribution system pressure drop,reducing

the specific fan power, which is the power required to move

a given quantity of air (Watts per cfm).

2. Enhanced opportunities for free cooling and evaporative

cooling.

3. Higher cooling equipment operating efficiency.

Fan Power

The specific fan power in an underfloor system is almost always

lower than in a conventional system since underfloor systems

Comparison of Mixed, Displacement and Hybrid Underfloor System Flow Rates

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Mixed System Hybrid SystemDisplacement System

SupplyFlow

Rate(CFM/SF)

Perimeter zoneCore zone

Flow rates for mixed,displacement, and hybrid underfloor systems were

calculated for a typical office space.While the flow rates increased for the

underfloor systems, the increases were fairly modest.2

Figure 5: Comparison of design flow rate for mixed,displacement, and hybrid underfloor systems



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typically require minimal ductwork on the supply side of the

system. The cross-sectional area of the underfloor plenum is

significantly larger than that of a conventional air duct, thereby

reducing air velocity and pressure drop. Horizontal supply ducts

are minimized or eliminated as the access floor serves todistribute the air evenly across the floor plate.Terminal boxes may

still be required, but diffuser branches can also be eliminated.

Typical static pressure design points for underfloor systems are

0.5 to 1.5 inches lower than for conventional systems,resulting in

a savings of 12 percent to 38 percent in specific fan power.3

Free Cooling

Underfloor systems generally deliver higher-temperature air to

the space when compared to overhead systems, so a greater

opportunity exists to take advantage of free coolingusing an air-

side economizer. As the setpoint of the supply air temperature

increases,warmer outside air can be used to meet the load,so the

hours required of the mechanical cooling equipment decrease.

Warmer return air temperatures due to stratification also allow an

increase in economizer operating hours.

Systems using evaporative coolers can also benefit in the same

manner. Just as economizer hours increase in a displacement

ventilation system, so do evaporative cooling hours. As the

setpoint for the supply air temperature rises, the evaporative

cooler can be expected to meet the load over a longer period of

the cooling season, without the need for mechanical cooling.

The impact of supply air temperature on the number of hours

that an economizer system can meet the load without

mechanical cooling is shown in Figure 6, page 10.

Enhanced Operating Efficiency

Another opportunity for energy savings lies in the increased

efficiency of the equipment used to mechanically cool the supply

air. Compared to a conventional system, an underfloor system

generally delivers warmer air to the space and typically returns


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warmer air to the equipment.As the supply air temperature rises,

the efficiency of the cooling equipment improves, since there is

less lift between the supply air and the condensing temperature.

The net effect is that the mechanical equipment serving an

underfloor air distribution system uses less energy than a

comparable conventional system serving the same cooling load.

However, one caveat may prevent the designer from fully realizing

this opportunity. In spaces with high latent loads,allowing a higher

equipment discharge temperature may result in an excessive

supply air dewpoint temperature.The air being delivered to the

space may not be sufficiently wrung outand comfort in the space

may be compromised. The designer may mitigate this potential

humidity problem without conceding equipment efficiency by

using a desiccant dehumidification system.

Access Floor Systems

Access floors are constructed from a series of panels elevated

above the original floor by pedestals located at the corner of

each panel.The access floor creates an underf loor space through

Figure 6: Economizer hours

Annual Hours Without Mechanical Cooling

0

500

1000

1500

2000

2500

3000

3500

CTZ 3Oakland

CTZ 6Long Beach

CTZ 7San Diego

CTZ 12Sacramento

CTZ 14China Lake

Climate Thermal Zone

HoursperYear

Dry bulb 55F Dry bulb 68F

The figure below shows the number of hours per year that an air-side economizer

can cool the building without mechanical cooling in several climate zones in

California. By raising the supply temperature from 55F to 68F, the number of

hours of free cooling increases dramatically.



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which power, network, and phone wires can be installed.With a

modest expansion in the depth of the underfloor space, a

plenum for distributing conditioned air can also be created.

Access Floor Design

Access f loor panels are available in four dif ferent materials:

all-steel; concrete; aluminum; or wood (Figure 8). All -steel

floor panels have a number of benefits.They are lightweight,

which allows for ease of handling; they have a high load

performance; and they are noncombustible. Concrete floor

panels are used in offices and equipment rooms. They have

an excellent rolling load performance and the panels are

solid and free from any floor- or plenum-generated noise.

They also have excellent grounding and electrical continuity.Aluminum panels, also known as floating floors, are used in

clean rooms and other high-tech locations.These panels are

unique in that they are perforated and have grates to provide

optimum laminar airflow patterns in a downflow

configuration, thus preventing particles from contaminating

Figure 7: Coefficient of Performance (COP) as a function of lift

Figure 8: Access floor panel

Source: Tate Access Floors

Chiller COP as a Function of Lift

0

2

4

6

8

10

12

0 5 10 15 20 25 30 35 40 45

Lift (deg F)

COP

The figure below shows influence of the lifton the COP of a centrifugal chiller.

Here, the lift is defined as the difference between the leaving chilled water

temperature and the entering condenser water temperature.

A typical access floor panel is shown in

the figure below. The structural panel is

supported by adjustable pedestals and

covered with a carpet tile.

Source: ASHRAE Standard 90.1999


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sensitive machinery and equipment in clean rooms. Floating

floors contain no ferrous material to interfere with magnetic

fields and are available in a wide selection of conductive,

static-dissipative coverings or coatings.Woodcore panels are

available as a lower-cost option for offices and equipmentrooms.They are durable, quiet, and economical, but building

codes do not allow them to be used with underfloor supply

systems because they are combustible.

Pedestals are located under the corner of each panel. A pedestal

consists of a base, tube, and head. The height of the pedestal

determines the height of the floor and the depth of the

underfloor space. The base is attached to the underlying

concrete slab or steel deck by either mastic or mechanicalfasteners.The head is attached to the corner of the panel using

screws or a proprietary engineered fastening system. The

pedestal heights are adjustable, allowing the installation of a

laser-leveled floor system over a concrete slab that may contain

irregularities or not be level. Once the access floor is installed

and leveled, subsequent leveling of furniture, bookcases,and file

cabinets is generally not required.

Access f loors can be finished in a number of dif ferentmaterials. In office environments, panels can be covered in

carpet, conductive and static-dissipative vinyl, wood, or high-

pressure laminate (HPL). Recycled carpet and cork-finished

tiles are also available as an environmentally responsible

alternative to conventional materials. In clean rooms with

floating floors, panels are finished in a variety of conductive

and nonconductive coatings.

Wiring Systems

An access floor simplifies the installation of power, network,

video, and phone wiring. The wiring systems are run at floor

level where they are needed, eliminating the necessity for

power poles to bring cables from the ceiling down to floor

level. Wire trays and cable support systems used above


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suspended ceiling systems are not required. Systems furniture

and demountable walls are no longer required to carry

electricity down from the ceiling, so they can be moved without

bringing in a electrician. Access to the cabling systems and

connection points is accomplished by simply lifting theappropriate floor panels.

A recent advance in cabling technology is the structured cabling

system. This system integrates the various cabling needsdata,

telephone, power, and building controlsinto a set of packaged

multifunction cables, complete with junction boxes and snap-in

connectors. Data,telecom,standard and isolated ground power is

brought to each workstation in a single, multifunction panel.

The systems can be extended while under load, maintainingproductivity of adjacent workstations during office reconfiguration.

HVAC Components

Access floors can also provide the plenum space necessary for

installing the air distribution components of an underfloor air

distribution system, such as supply air diffusers, terminal units,

and optional ductwork. Plenum systems can be either

pressurized or operated at neutral pressure. Pressurized

plenums generally use passive diffusers; neutral-pressure

plenums use fan-powered diffusers. Fan-powered diffusers

provide better local control of airflow rate at the expense of

additional fan power, noise and cost.

Displacement ventilation systems use diffusers with a sufficient

cross-sectional area to limit air velocity to about 40 to 50 fpm.

Low-velocity diffusers are available from several manufacturers

for use in access floor systems.Airflow rate can be adjusted by

the occupants, providing some level of local occupant comfort

control. Diffuser location is not particularly critical, since the

exit velocity is designed to limit localized draft discomfort.

In hybrid underfloor systems, floor-mounted diffusers are

intended to induce room air into the discharge airstream.

A structured cabling system integrate

power, data, telecom, and video into a se

of multifunction cables, connectors, and

terminal boxes.

Two types of HVAC terminals used

with access floor systems are pictured

below. The top terminal is a

thermostatically controlled VAV

terminal; the lower terminal is a

constant-volume swirl diffuser.

Figure 9: HVAC terminals

Source: Tate Access Floor


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Typical diffuser designs include manually adjustable 8-inch-

diameter, circular Swirl-type models, as well as manually or

automatically adjusted, 10 x 10-inch constant-velocity models.

The swirl-type models can be installed in 8-inch-high raised

floors, while the constant velocity models require 12-inchplenums. Most manufacturers designs allow occupants to

manually regulate the flow through floor-mounted, swirl-type

diffusers without removing the diffuser cover. Another style of

diffuser allows easy adjustment of flow direction by simply

rearranging the supply grilles in one of 16 different

orientations. Some floor-mounted devices can be electronically

dampered through a locally controlled thermostat.

More elaborate configurations are designed to be mounteddirectly on the desktop, affording the owner of the workstation

arms-length control of the quantity and direction of flow at

head level. These diffusers are typically incorporated in "task-

ambient conditioning" (TAC) systems, in which underfloor air is

extended through the floor in flexible ducts housed directly in

the furniture. The diffuser, akin to that found on an airplane

above the passengers head, is mounted in a desktop console

with other environmental control apparatus, such as light and

speaker dimmers or switches.

Maintenance and Cleaning

Maintenance of an access floor system is straightforward.Due to

the modular nature of the system, carpet can be replaced as

needed in high-wear areas simply by replacing the carpet on the

affected sections. HVAC diffusers have a removable bucket trap

that captures dirt and spills before they enter the underfloor

space.These traps are emptied during normal janitorial service.

Comfort Control and Productivity

Improved comfort can be achieved when the occupants

themselves take advantage of the local control features

available with an underfloor system. Uncomfortable draft and


A task ambient control workstation

allows the occupant to have complete

control of the local lighting, HVAC,

and ambient sound environment.


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temperature inconsistencies are two of the most common

sources of occupant complaints. With the diffusers at floor

level, the occupants can easily redirect or modulate the airflow

into their own space and to their own liking, a benefit that is

inconceivable with the conventional overhead system.Furthermore, since the diffusers are housed in a modular

access floor, it is easier to reposition the diffuser to another

location. Comfort is also improved by the reduction in

inherent noise levels for pressurized floor systems. Since they

operate at lower pressure and velocity, underfloor systems

generally produce less noise than traditional overhead

systems. However, fan-powered terminals used in neutral-

pressure systems may cause a small increase in noise levels.

We have far fewer temperature complaints, no indoor air

quality problems, and much happier occupants, says Gary Hall,

director of facilities at the Gemological Institute of America in

Irvine,California.

Glenn Shuder, principal architect of the Sacramento Municipal

Utility Districts Customer Service Center, says, The greatest

benefit is that our employees have control over their

environment and have not placed individual heaters at theirdesks, due to the vents at the floor and not the ceiling.

Comfortable employees are productive employees. The

connection among air quality, comfort, and productivity is

commonly addressed through owner testimonial letters as

well as various articles published in trade magazines. Space

comfort and air quality are the top occupant complaints

fielded by building owners and operators, according to the

Building Owners and Managers Association (BOMA). Studiesby BOMA point to potential increases in worker productivity

of 20 percent by improving indoor air quality. When

occupants are satisfied with the temperature and quality of

the air in their immediate environment, they spend less time

fretting and more time working.



16/28

Simply giving occupants control of their personal comfort

goes a long way toward meeting this end.The advantages of

personal comfort control are documented in a study of the

West Bend Mutual Insurance Companys new energy-efficient

headquarters (Figure 10). The new building has efficientlighting, high-quality windows and shell insulation, and an

efficient thermal storage HVAC system. The designers also

included unique personal HVAC control features for 430 of the

500 employees.The results of the study estimated an overall

increase in worker productivity of 16 percent after the move

into the new building. This speaks volumes to the general

advantages of energy-efficient design. With respect to the

HVAC system in particular, the incremental increase in

productivity was roughly 3 percent, which translated to an 18-

month payback on the personal control modules. The

investigators took this to be an underestimate, since many of

the employees reneged on their commitment to the study

when they learned they would temporarily lose control over

their personal modules.


A survey of building occupants listing their concerns and level of satisfaction.Comfort, indoor air quality, and occupant control of the space temperature are

highly ranked issues.4

0

10

20

30

40

50

60

70

80

90

100

Comfortabletemperature

Indoor airquality

Tenant control oftemperature

Image andprestige ofbuilding

Cost of after-hours heatingand cooling

Proximity tobusinessservices

Proximityto where

employees live

PER

CENTAGE

Percentage Considering It Important Percentage Satisfied

Figure 10: BOMA occupant satisfaction survey

Source: Building Owners and Managers Association


17/28

Economics

The basic cost of an access floor system is on the order of $4-7

per square foot. Savings from reduced wiring and HVAC costs

during the first tenant improvement bring the net cost down to

about $3 per square foot. Access f loors inherently reduce

ongoing operating costs in spaces with a high churn rate,where

people, furniture, and equipment are frequently moved to

accommodate changing project or team requirements. Facilities

managers can spend less time and money on renovations, since

changes in wiring and HVAC services are simple and

uncompromising. If cable or HVAC outlets are out of reach or

inconveniently located, they are simply moved.

Taken together as an integrated system, the underfloor air

distribution system with an access floor can afford the building

owner an overall decrease in life-cycle cost. Savings in space

remodeling costs over the life of the system can easily pay for

the incremental cost. Estimates of savings in ongoing

operations, maintenance, and remodeling are shown inTable 1.

Like the floor itself, underfloor HVAC components may be

depreciated at a faster rate since they are considered equipment

rather than part of the building. In the balance, higher


Reconfiguring an access floor system

is accomplished by removing the

carpet tiles and unbolting the access

floor panels, thereby gaining access to

the wiring below. Electrical outlets,data connections, and HVAC terminals

can be moved to suit the new

configuration of the space.

Table 1: Energy- and non-energy-related cost savings for accessfloor systems with underfloor air distribution5

Category Estimated Savings

HVAC Energy 1530%

Operations and facility staff reduction 2550%

Cost of telecommunications moves, adds, and changes 4070%

Floor plate modifications cost 4070%

Power distribution changes to individual workstations 5080%

Moving computers and peripherals 8090%

Space unavailable during remodeling 3070%

Absenteeism 510%

Employee disruption and dead time during remodel 5080%

Source: Journal of the International Facilities Management Association


18/28


installation costs associated with the access floor can be offset

by lower first costs in wiring and HVAC system costs, lower

operations and maintenance costs, and higher occupant

satisfaction. Finally, with flexibility and accessibility come the

advantages of adaptability.Buildings served by raised f loor systemshave the advantage of deferred obsolescence. As they are more

adaptable to changes in wire and HVAC service requirements, they

can maintain market value over a longer period of time.

Applications

Certainly any spaces that are designed with the open office

plan are good candidates for underfloor air. Whenever high

employee churn rate is an inherent characteristic of the space,

the underfloor system is a natural choice.As occupant loads are

frequently added,subtracted, shuffled,and moved,the f lexibility

and accessibility of the underfloor system serve as a great

financial benefit to the building owners and operators, saving

time and money with each renovation.

Spaces with high ceilings provide good energy-savings

opportunities for designers of underfloor systems.Although 8

feet is considered to be the minimum ceiling height, a higher

ceiling promotes the formation of a large vertical temperature

gradient,producing the stratification layers that naturally induce

warmer stale air to rise above the cooler fresh air.

Examples of high ceiling spaces where underfloor supply and

displacement ventilation systems may be successfully

implemented are theaters, courtrooms, lobbies, and industrial

and manufacturing facilities. Spaces with low sensible loads in

the occupied zone also provide good opportunities for

designers of underfloor systems because they inherently

require less airflow and therefore pose less risk of a draft.

Energy-efficient daylit spaces represent a good opportunity for

underfloor supply systems since they often extend to the

designer the advantages associated with both higher ceilings

Underfloor air and access floor

systems have been applied to a widevariety of building types and climates,

including a corporate headquarters

for a major company in California,

an electric utility headquarters in

Sacramento, and a federal courthouse

in Denver (under construction).

Figure 11: Examples ofbuildings withunderfloor airsystems

Source: GAP Inc./SMUD/Anderson Mason Dale Architects


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and lower loads. Light shelves and interior shading devices are

generally located above the occupied zone. Solar heat gains

intercepted by these features do not contribute to the occupied

zone cooling load.Energy-efficient buildings with well-insulated

walls and high-performance glass have reduced cooling andheating requirements at the perimeter.

Other good candidates for underfloor air systems are industrial

and manufacturing facilities where heat-producing processes

like soldering, brazing, welding, grinding, and machining can be

served very effectively by a displacement system. In

Scandinavia, displacement ventilation systems account for

upwards of 50 percent of industrial market share.

Applications that may not work well with displacement

ventilation are those in which the pollutant sources are heavier

than air and not accompanied by heat. Some chemical and

biological manufacturing or laboratory spaces, for example, may

not lend themselves to successful implementation of

displacement ventilation systems.

Design Considerations

Temperature Gradient

In order to maintain occupant comfort, underfloor air systems

must limit the temperature gradient in the occupied space. In

order to limit the gradient, supply air temperatures on the order

of 65F to 68F are used with a recommended minimum ceiling

height of 8 feet. The supply air must be introduced to the

occupied lower space at a high enough flow rate to prevent an

uncomfortable temperature gradient between head and foot,

but at a low enough velocity to prevent the sensation of draftand prevent mixing in the upper stratified zone.A temperature

gradient between head and foot that is greater than 5F is

considered excessive. A gradient of 3.5F is considered to be a

good design criterion, not too large by most peoples standards.

The trade off between system flow

rate and temperature gradient is one

of the most important decisions facing

the designer of an underfloor ai

distribution system.


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Airflow Rate

The airflow rate must be balanced against temperature gradient

considerations. Lower temperature gradients imply higher flow

rates and excessive fan energy. If the air volume is too high,

occupants are more likely to be exposed to drafty conditions

resulting from high-velocity currents discharging into the

occupied lower portion of space.Conversely, if the air volume is

too low, the occupants are more likely to be exposed to

uncomfortable temperature gradients between their ankles and

head. The high load problem may be mitigated in applications

where sufficient floor area is available to supply high volume

through more or larger diffusers, effectively reducing the

localized air velocities while maintaining a high limit ontemperature gradient. Installing diffusers that give the occupant

the benefit of local control may also mitigate the high load

problem, as they allow immediate adjustment of both velocity

and direction of flow.

Combined Underfloor and Conventional Systems

Designers need to be cautious in buildings where the plan calls

for the parallel installation of displacement and overhead

systems.Technically it would be a fairly simple matter to design

this hybrid underfloor system with a common plant and air

handler. However, because the energy-saving features of the

displacement system hinge on the production of higher-

temperature supply air, many energy savings opportunities are

lost when combining underfloor and overhead systems on the

same air-handling system or plant.

When combining air -handling systems, the supply air

temperature is set according to the needs of the overhead

system. For spaces served by underfloor systems, some type of

zone level mixing is generally provided (e.g., fan-powered

mixing boxes) to warm up the supply air before it can be

supplied to the space, since it would be too cold for direct

injection. Fan-powered mixing boxes increase first costs and fan


Designers of buildings with both

underfloor and overhead air distribution

systems should consider using separate

air handling and mechanical cooling

systems dedicated to each air

distribution system type, to avoid

compromises that reduce the energy

efficiency of the underfloor system.


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energy and reduce the effectiveness of the air-side economizer.

Another problem with combining air handlers is the difference

in pressure required. Conventional VAV systems require

significantly higher supply pressures than underfloor systems.

Combining them requires a throttling device to reduce thepressure before supplying air to the underfloor plenum, which

wastes fan energy. To avoid this compromise, the designer

should consider separate air-handling systems for each type of

distribution system.

Serving both conventional overhead and underfloor systems

with the same chiller plant can be both problematic and

inefficient. Since the overhead system supplies colder air, the

overhead air handlers will require that the cooling plant operatein mild weather when the underfloor air handlers do not need

cooling. This will increase the number of hours the plant

operates, and it may cause the plant to operate inefficiently if

loads are very low (e.g. if the conventional air handler load is

small relative to the total plant load). Chilled water supply

temperatures will also have to be lower for the overhead

systems due to their low supply air temperatures,reducing plant

efficiency under all load conditions.

Plenum Conditions and IAQ

While underfloor systems are generally advertised as offering

an improvement in indoor air quality, consideration must be

given to the possible effects of condensation buildup in the

plenum. Condensation can be a concern in underfloor systems

if the cool plenum is suddenly exposed to warm, moist air.The

dewpoint of the air entering the plenum must always be

greater than the lowest temperature of any exposed surfacewithin the plenum, otherwise condensation will occur. This

phenomenon can be avoided if the plenum is well sealed

against outside air infiltration, and if sudden step-changes in

supply air conditions are avoided. For example, when the fan

shuts down at the end of the day, warm and moist air should be


Care must be taken in the design of

HVAC control sequences to avoid

unintentional moisture condensation in

the underfloor space.


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blocked from rushing into the cool plenum. Similarly, during

occupied hours, if the mechanical cooling abruptly shuts down

and the dewpoint of the supply air suddenly rises, the fan may

need to be shut off, or the outside air dampers adjusted.

Condensation concerns need particular attention when theslab thermal reservoir is used to cool the space. For example,

after the slab is charged with cool air at night, the mechanical

system must be controlled to ensure that it does not supply air

that exceeds the temperature of the coolest surface in the

plenum. This can be a problem with systems installed over a

slab on grade, since the slab temperature will be difficult to

control, and condensation can occur.

Some questions may also arise regarding the inherent quality ofthe air that passes through an underfloor plenum prior to

discharging into the space. Off-gassing from cabling, cement

dust shed from the slab, debris and spills sifted down through

the floor, and biological growth due to moisture are all possible

sources of pollution that could degrade the quality of the

ventilation air before it even enters the space. Allowances for

these ill effects need to be considered during design of the

system. Floor panels can be manufactured and installed to close

tolerances, diffusers can have traps that capture spills and

debris and can be easily cleaned, and slabs can be sealed to

reduce dust and inhibit bacterial growth.

Impacts on Building Design

In most applications, floor-to-floor heights are not impacted,

because the depth of the ceiling plenum can be reduced when

an access floor system with underfloor air distribution is used.

Locations of return air grilles are not critical so long as they arelocated near the ceiling level. With careful consideration of

return air system and lighting design, it may be possible to

reduce floor-to-floor heights while maintaining the same floor

to ceiling dimension.This is particularly true for buildings that

do not have hung ceilings, a design that is made practical by the


Floor-to-floor heights need not increase

when using an access floor system.


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elimination of overhead ductwork and wiring that the ceiling

was intended to conceal. Local fire codes may place conditions

on running wire in an HVAC plenum. Fire-rated wiring and/or

conduit will be required in almost all jurisdictions.The plenum

size may also be limited by the fire code, so it may be necessaryto partition the plenum into smaller subspaces. It may also be

necessary to install smoke detectors in the plenum.

Perimeter Systems

Perimeter systems are required to respond to time-varying heat

gains and losses through the building shell. While occupant

control of relatively constant interior loads is reasonable,

manual control for the ever-changing perimeter loads is not,and

therefore underfloor plenums may need to be zoned based on

building exposure, especially when using passive diffusers.The

plenums can be partitioned and fed with separate supply air

ducts as needed. In hybrid underfloor systems, fan-powered VAV

terminal boxes can provide extra cooling to the building

perimeter. These boxes can be supplied with integral heating

coils to provide perimeter heating as well. Displacement

ventilation systems do not function well in the heating mode.

Generally, separate perimeter radiation is provided to meetenvelope heating loads.

Design Methods

Underfloor air distribution systems involve a considerably

different process than with overhead systems when calculating

space load, supply flow rate, and temperature, along with

selecting the size and number of diffusers. A variety of methods

are available for finding the optimal design values, some more

complex than others. The type of underfloor system being

installed, displacement ventilation or hybrid, plays a role in

choosing the design methodology. The most complex and

rigorous design methods usually involve use of computerized

computational fluid dynamics (CFD) analysis,which is generally

reserved for research applications or very large projects with


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unique conditions.Simplified design methods have been developed

to assist the designer with more mainstream design problems.

Displacement Ventilation Design

A simplified method for displacement ventilation design hasbeen developed by ASHRAE, which is based on CFD analysis

but does not require CFD in its implementation.6 The method

uses empirical formulas derived from CFD models of

prototypical office and classroom spaces, and predicts

occupied zone cooling load, temperature gradient,and system

flow rate from the occupied zone heat sources (occupants,

equipment, and desk lamps), overhead lighting, and exterior

loads (envelope conductance and solar gains). Empirically

derived weighting factors are used to estimate the occupied

zone load from these components of the total space load.

System flow rate is calculated assuming a maximum

temperature gradient of 3.5F in the occupied zone. Empirical

formulas are also provided to calculate the ventilation

effectiveness and other IAQ parameters.

Hybrid Underfloor System Design

A simplified design method has also been developed for hybridunderfloor systems, called the Effective Sensible Heat Gain

(ESHG) method.7 In a manner similar to the displacement

ventilation design method, the sensible load between the head

and foot region is estimated by applying weighting factors to

the components of the total zone load. System flow rate is

calculated from the occupied zone load and the maximum

allowable differential in the supply air and occupied zone

temperatures.The weighting factors used in the ESHG method

are based on engineering judgment. Research being conducted

at the Center for the Built Environment at UC Berkeley will

result in a more rigorous design method for these systems.



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For More Information

American Society of Heating, Refrigeration, and Air Conditioning Engineers

(ASHRAE)

ASHRAE is organized for the sole purpose of advancing heating, ventilation, air

conditioning, and refrigeration for the publics benefit through research,

standards writing, continuing education, and publications.

1791 Tullie Circle, NE

Atlanta, GA 30329

Phone: (404) 636-8400

Fax: (404) 321-5478

www.ashrae.org

1997 ASHRAE Fundamentals Handbook, pg. 31.531.8. 1997.

A Critical Review of Displacement Ventilation. Xiaoxiong Yuan, Qingyan Chen,

and Leon R. Glicksman. Vol. 104, Pt. 1. 1998

Assessment of the Performance of Ventilated Floor Thermal Storage Systems.

Michael J. Holmes and Alexandra Wilson. 1996.

Comparison of Heating and Cooling Energy Consumption by HVAC System with

Mixing and Displacement Air Distribution for a Restaurant Dining Area in

Different Climates. Alexander M. Zhivov and Adolf A. Rymkevich. Vol. 104, Pt.

2. 1998

Development and Application of an Underfloor Air-Conditioning System with

Improved Outlets for a Smart Building in Tokyo. Katashi Matsunawa, Hiroshi

Iizuka, and Shin-ichi Tanabe. 1995.

Experimental Study on the Floor-Supply Displacement Ventilation System.

Takashi Akimoto, Tatsuo Nobe, and Yoshihisa Takebayashi. 1995.

Integrated Access Floor HVAC: Lessons Learned. R.W. Shute. 1995.

"Measurements and Computations of Room Airflow with Displacement

Ventilation." Xiaoxiong Yuan, Qingyan Chen, Leon R. Glicksman, Yongqing Hu,

and Xudong Yang. Vol. 105, Pt. 1. 1999.

Underfloor Air Distribution Solutions for Open Office Applications. Kenneth J.

Loudermilk. Vol. 105, Pt. 1. 1999.

Underfloor Air Distribution Systems: Benefits and When to Use the System in

Building Design. Blair T. McCarry. 1995.



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Engineered Systems.

Issues & Events. September 1999.

HPAC Engineering.

Design for Commercial, Industrial, and Institutional Facilities. William A.

Turner. October 1999.

Journal of the International Facilities Management Association.

Access FloorDont Plan Your Building Without It. Theodore R. York.

Nov./Dec. 1992.

University of California at Berkeley

390 Wurster Hall

Berkeley, CA 94720

Phone: (510) 642-4950

Fax: (510) 643-5571

www.cbe.berkeley.edu

Center for the Built Environment. How Low Can You Go: Air Flow

Performance of Low-Height Underfloor Plenums. Fred Bauman and Paolo

Pecora. October 1999.

Center for the Built Environment. Giving Occupants What They Want:

Guidelines for Implementing Personal Environmental Control in Your Building.

Fred Bauman. October 1999.

Center for Environmental Design Research. Task/Ambient Conditioning

Systems: Engineering and Application Guidelines. Fred S. Bauman and EdwardA. Arens. October 1996.



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Design Brief-Underfloor Air Distribution

Documents